Abnormally high currents or over currents have the potential to destroy electrical circuitry and equipment plus become a safety/shock hazard to people. For years electrical equipment has been protected through the use of fuses or circuit breakers. The typical fuse has an internal by-metallic conductor through which current passes. If the current through the system exceeds the rated value of the fuse, the by-metallic conductor will begin to melt. If the over current continues, eventually the by-metallic conductor will melt through thereby breaking the current path between the supply and the load. Circuit breakers break the current path between the power source and the load just as the fuse except that in the case of the circuit breaker an electromagnet in the device draws a connecting metallic link out of the circuit when current levels exceed rated conditions thereby opening the current path.
While typical fuses are effective in protecting electrical equipment they must be replaced once the by-metallic conductor has melted thereby proving not to be cost effective, both with regard to replacement and maintenance costs. Circuit breakers on the other hand are reusable but are typically more expensive than fuses and still require user intervention to reset them once an over current has been corrected.
An improvement over by-metallic fuses and circuit breakers is the polymeric (positive temperature coefficient) PTC device which protects circuits by going from a low to a high resistance state in response to an over current. Polymeric PTC devices respond to over currents by increasing their resistance as the device""s temperature increases due to the generation of heat within the device from power dissipation. The advantage these devices provide is that once the over current condition has been corrected and the devices temperature decreases to its normal operating point, its resistance will decrease in effect resetting the device. Polymeric PTC devices provide the compact dimensions provided by fuses with the ability to be reused as provided by circuit breakers. These devices have the added advantage of being automatically reset once normal operating conditions in a circuit are restored.
To date, polymeric PTC devices have been manufactured in standard electronic packaging, similar to a disk capacitor, in which the polymeric material is encapsulated in a disk shaped enclosure with two wire leads extending therefrom. This type of packaging is designed for through hole circuit board mounting. While this configuration protects circuitry from over current conditions, the through hole wire leads and associated copper tracks and wiring used to connect the polymer PTC devices cause another problem, that being radiated emissions of electromagnetic noise.
The wire leads and copper tracks used by the polymeric PTC devices are effected by two different types of conductive currents, differential mode and common mode. The fields generated by these currents create the radiated emissions. Differential mode currents are currents which flow in a circular path in wires, copper traces, and other conductors such that the fields associated with these currents originates from the loop defined by the conductors. Such a circuit is essentially a loop antenna with the resulting field being primarily magnetic. Differential mode emissions are typically found at frequencies below 1 MHz.
Common mode currents are completely different in nature from differential mode currents in that they flow in a different circuit path and dominate at higher frequencies, those typically above 1 MHz. Common mode currents typically return to their source through parasitic capacitance inherently found in electronic circuits. To minimize common mode currents between lines, an alternate low impedance return path for these currents must be provided while increasing the impedance of the common mode current path.
A major drawback to differential and common mode filters of the prior art is that if capacitors failed they would adversely affect the circuitry they were originally used to filter or protect. Dependent upon the application, this condition would also present a safety hazard to humans. To overcome this problem numerous fuses had to be employed in various configurations to ensure the electrical conductors to be filtered were protected from both the filter and other circuitry. Despite the numerous fuses and inconvenience of using so many fuses, in many applications it was absolutely necessary to prevent inconvenience or even life threatening conditions. One example is the use of filters in the automotive industry. When used to filter differential and common mode electrical noise from conductors in power steering or power brakes, filtering improves the overall operation but is not critical to this operation but if the filter failed and destroyed surrounding other circuitry, the brake or power steering systems could become disabled either stranding or endangering the driver if failure occurred while the vehicle was moving. Because of this it is critical that any filters used be self protecting in that they effectively remove themselves from circuitry they were originally intended to protect upon their failure.
As a result, one object of the present invention is to provide in-line and/or bypass fuse protection in a single, compact electrical device which allows defective circuitry to disconnect or remove itself from protected circuitry while not presenting a safety hazard.
It is a further object of the present invention to provide a compact hybrid device which combines various types of filter and surge protection with in-line and/or bypass fuse protection to improve overall circuit performance and insure that under extreme conditions partial circuit failure does not damage additional circuitry.
It is an additional object of the present invention to provide a filter apparatus capable of removing itself from additional circuitry upon its own failure such that continued operation of the overall system can be maintained in spite of the filter""s failure thereby providing additional safety when used in conjunction with systems whose failure can endanger human life.
Therefore, in light of the foregoing deficiencies in the prior art, Applicant s invention is herein presented.
The present invention relates to a polymer fuse apparatus that provides bypass fuse protection. A polymer bypass fuse is comprised of an electrical conductor wherein a portion of the conductor is surrounded by an internal electrode, which is then surrounded by a layer of polymeric positive temperature coefficient (PTC) material, which is then surrounded by a conductive material similar to that of the internal electrode.
During normal operation the polymeric PTC material is in a conductive state thereby allowing electrical coupling between the conductor and the outer conductive material or contact electrode. If the current through the polymeric PTC material increases beyond acceptable limits, the polymeric PTC material will become highly resistive thereby creating an open condition which prevents conduction. Various hybrid combinations are also contemplated where in-line and/or bypass fuses are combined with other circuit components. One example is, a plurality of in-line and bypass fuses combined with a differential and common mode filter, which itself consists of a plurality of common ground conductive plates maintaining first and second electrode plates between the various conductive plates, all of which are surrounded by a material having predetermined electrical characteristics to provide filter and circuit protection.